Prepare subpopulations of candidate target cells from C57BL/Ka mouse thymus, bone marrow, and spleen. Characterize these subpopulations with respect to a series of differentiation-related markers. Detection of lymphocyte related antigens (O(Thy-1)), TL, Ly, GIX, and Q. Determination of the intensity of H2 antigen expression. Capacity for binding histamine. Sensitivity to cortisone and related steroids. Presence of differentiation related enzymes. Study the interaction of Rad LV and candidate target cell subpopulations as revealed by infectivity and neoplastic transformation in vitro and by altered properties of the infected target cell populations in vitro and in vivo. Analyze the state of differentiation of infectable subpopulations from bone marrow and thymus. Assay infection of target cells by RadLV. Identify surface antigen phenotypes of transitional forms of malignant transformation. Assay for neoplastic transformation of target cells after infection.